GHC Status October 2011

GHC is still humming along, with the 7.2.1 release (more of a "technology preview" than a stable release) having been made in August, and attention now focused on the upcoming 7.4 branch. By the time you read this, the 7.4 branch will have been created, and will be in "feature freeze". We will then be trying to fix as many bugs as possible before releasing later in the year.

We advertised 7.2 as a technology preview, expecting 7.4 to merely consolidate the substantial new features in 7.2. But as it turns out GHC 7.4 will have a further wave of new features, especially in the type system.
Significant changes planned for the 7.4 branch are:

Declarations at the GHCi prompt. Simon Marlow has extended GHCi so that it is possible to give any declaration at the ghci prompt. For example,

Prelude> data D = D Int
Prelude> case D 5 of D x -> print x
5

This has already been merged, so will definitely be in 7.4.

Data type promotion and kind polymorphism. As we do more and more type-level programming, the lack of a decent kind system (to make sure that your type-level programs make sense) has become an increasingly pressing issue. If all goes well, GHC 7.4 will take a substantial step forward:

Associated type synonym defaults. Haskell lets you give a default method for the operations of a class.
Associated type synonym defaults let you declare a default type instance for the associated type synonyums of a class. This feature, implemented by Max Bolingbroke, nicely fills out missing design corner. For example

Since we do not give a definition for T in the instance declaration, it filled in with the default given in the class declaration, just as if you had written type T Int = [Int].

Monad comprehensions. After a long absence, monad comprehensions are back, thanks to George Giorgidze and his colleagues. With {-# LANGUAGE MonadComprehensions #-} the comprehension [ f x | x <- xs, x>4 ] is interpreted in an arbitrary monad, rather than being restricted to lists. Not only that, it also generalises nicely for parallel/zip and SQL-like comprehensions. The aforementioned generalisations can be turned on by enabling the MonadComprehensions extension in conjunction with the ParallelListComp and TransformListComp extensions.

Rebindable syntax is fully supported for standard monad comprehensions with generators and filters. We also plan to allow rebinding of the parallel/zip and SQL-like monad comprehension notations.

Improvements to the implementation of type constraints. Over the last six months, Dimitrios and Simon PJ (with Stephanie Weirich and Brent Yorgey) have figured out several improvements to the GHC's type constraint solver and its strongly-typed Core language. The changes to the constraint solver eliminate hundreds of lines of code, and make it more efficient as well. The changes to the Core language make it treat equality constraints uniformly with other type constraints; this makes the internals vastly more uniform. These changes are mostly invisible to programmers, but the changes to Core allow us to support equality superclasses for the first time. Details in our paper "Practical aspects of evidence-based compilation in System FC" ​http://research.microsoft.com/~simonpj/papers/ext-f/ NewFC

Profiling and hpc overhaul. GHC currently has three different ways of tracking which pieces of code are executed: const-centre profiling, HPC coverage, and GHCi debugger breakpoints. Each is implemented a different, anf somewhat ad hoc way. Simon Marlow has overhauled the whole system, unifiying the three mechanisms into one. On the way he has improved the semantics of cost centre stacks, which should lead to more useful time and space profiles.

Changes to the way Safe Haskell works David Terei has improved the design of Safe Haskell since the 7.2.1 release. In particular, it will no longer cause build failures for users who do not explicitly enable it. David to fill in.

Joining in

We continue to receive some fantastic help from a number of members from the Haskell community. Amongst those who have rolled up their sleeves recently are:

Ben Gamari, Karel Gardas and Stephen Blackheath have been working towards getting a registerised Arm port working

Liquid types

Ranjit

Parallel project

Duncan

DPH

Manuel

The glorious future

Also on the go, but not yet fully baked, are:

Work on adding contracts to Hsakell, along the lines of Dana Xu's these, but using a first order logic theorem prover to check contract satisfaction (with Koen Claessen, Dimitrios Vytiniotis, Charles-Pierre Astolfi, and Nathan Collins).